QAM modulation techniques are increasingly used to obtain maximum bandwidth efficiency in many communications systems. Such systems encode data by assigning data strings to one of a plurality of points in a signal constellation map in the complex plane.
A problem encountered in all such systems relates to the resolution of quadrant ambiguity resulting from carrier recovery. The receiver has no way of determining which quadrant of the signal constellation the transmitter has used to send a received point. Traditional modem techniques employ differential encoding, by assigning two bits of each data string to define the absolute quadrant. The remaining bits, for example 8 in a 1024 QAM system, are used to define the point within the quadrant. This is illustrated in FIG. 1.
However, there are still difficulties in determining the absolute bit pattern for the received quadrant. In order to overcome ambiguity due to rotation, the points are generally coded such that they incorporate rotational symmetry.
U.S. Pat. Nos. 4.601,044 to Kromer etal, and 4,416,016 to Iapicco disclose techniques for extracting phase information from trellis or convolutional encoded modulations. U.S. Pat. No. 4,866,395 to Hostetter discloses a system for carrier recovery. None of these disclosures addresses the issue of avoiding the requirement for differential encoding.
Conventional design methodologies dictate that differential encoding is essential to solve the phase ambiguity problem.
However, differential encoding in turn creates problems which become particularly evident in a large, for example 1024 point, QAM system. Gray coding on quadrant boundaries is very poor, and can lead to unacceptable error bursts when coupled with differential decoding errors.
It is an object of the present invention to provide a modem design and encoding system which ameliorates the deficiencies of the prior art.